Whenever I've had letters from the Pension service, they typically get to me 10-14 days after the date on the letter. However, letters written by normal people typically take 2-3 days. I suspect the Pension service uses a bulk mailing system which dispatches the mail at irregular intervals, which is why it takes so long.

Very much my suspicion too, that the date they write the letters is some way off the date they're posted. Similarly with the date they receive/read inbound letters.

...and....

the certificates arrived today. Only 13 working days...

I just received today a couple of letters from a UK public office dated 8 April but no idea when they were posted.